1. Field of the Invention
This invention relates to an automatic focusing device for use in video cameras of varied kinds and the like.
2. Description of the Related Art
The recent advancement of video apparatuses such as video cameras is conspicuous. They are now equipped in general with devices for automatic focusing, automatic iris control and high-magnification zooming and further with an electronic zoom function of electrically magnifying images by image processing actions. These apparatuses thus are being arranged to have improved operability in every part as well as proliferated functions.
With respect to the automatic focusing device, it is becoming a main trend to arrange the device for focus adjustment to detect the sharpness of a picture from a video signal obtained by photo-electrically converting an object image with an image sensor and by controlling the position of a focusing lens so as to reach a position where the sharpness takes a maximum value.
It is generally practiced to evaluate the image sharpness through the level (a focus evaluation value) of a high frequency component of a video signal extracted by a band-pass filter of a certain frequency band.
In taking a picture of an ordinary object of shooting, for example, the level of the high frequency component, i.e., a focus evaluation value, increases accordingly as focus on the object of shooting obtained by the position of the focusing lens being adjusted comes nearer to an in-focus state, as shown in FIG. 2. A position at which the level of the high frequency component takes a maximum value is considered to be an in-focus point.
In the process of focusing, the focusing lens is moved by a predetermined amount and, if the focus evaluation value then becomes smaller than the value obtained at the last position of the focusing lens, the focusing lens is made to be reversely moved. On the other hand, if the focus evaluation value becomes larger than the value obtained at the last position of the focusing lens, the focusing lens is made to be moved further in the same direction. The in-focus state can be maintained by repeating the above operations.
In making focus evaluation in this manner, in order to avoid the evaluation being affected by a noise, the focus evaluation value is considered to have decreased only when it decreases at least by a predetermined value. This predetermined value is called a reversal threshold value.
FIG. 3 shows how the focus evaluation value is extracted from an image pickup plane. In FIG. 3, reference numeral 301 denotes an image pickup plane and reference numeral 302 denotes a focus-evaluation-value extracting range. In this case, a peak value h1, h2, h3, h4 or h5 is obtained on each of scanning lines in the horizontal direction. A value which is obtained by adding up these peak values included within the range 302 in the vertical direction is used as a focus evaluation value H (H=h1+h2+h3+h4+h5).
FIG. 4 shows how the focus evaluation value is extracted from an image pickup plane magnified two diameters, i.e., enlarged two times, by an electronic zoom function. Reference numeral 301 denotes an image pickup plane obtained prior to being magnified, reference numeral 401 denotes an image pickup plane to be magnified and reference numeral 402 denotes a focus-evaluation-value extracting range.
If the picked-up image plane is magnified by the electronic zoom function without varying the focus-evaluation-value extracting range, the focus-evaluation-value extracting range would become larger than the image pickup plane, so that there is a possibility of causing an in-focus part to be located outside of the image pickup plane.
Therefore, it is conceivable to arrange the image pickup apparatus to be capable of making the focus-evaluation-value extracting range smaller, as shown in FIG. 4, when the image pickup plane is magnified by the electronic zooming or the like, so that focus adjustment can be reliably made without having the in-focus part outside of the image pickup plane even during the electronic zooming.
However, the arrangement mentioned above still remains to be improved further. Since the focus evaluation value is obtained by adding up the peak values of the focus evaluation values of horizontal scanning lines included within the range in the vertical direction, a reduction of the focus-evaluation-value extracting range during the process of electronic zooming as shown in FIG. 4 causes the focus evaluation value H to become "H=h1+h2+h3" which is smaller than an evaluation value normally obtained.
Therefore, in such a case, the reversal threshold value which is set for the larger normal evaluation value becomes too large and thus causes a point of time at which the driving direction of the focusing lens is to be reversed to become too late. As a result, an image thus picked up appears in an unstably focused state.